Problem Statement: Gaining reasoning skills in early years affects the formalproving skills in the following years, thus it is quite significant. Theacquiring of this skill is only possible with the approaches that theteachers used in the process. At this point, the problem to be researched interms of making proofs is seen in how middle school students prove amathematical expression; what kinds of reasoning and proof types theyuse in this process; how the teachers of these students prove the sameexpression; and how they reflect it to their instruction.Purpose of the Study: The purpose of this study is to investigate the middleschool students’ and their teachers’ reasoning types and proof methodswhile proving a mathematical expression.Method: A basic qualitative research design was conducted to investigatethe research problems. Participants in this study were two middle schoolmathematics teachers who have different professional experiences, and 18students from 6th, 7th and 8th grades. A clinical interview technique wasused to collect data and the interviews were video recorded. A thematicanalysis method was used to analyze the data.Findings and Results: The middle school students tried to decide on theargument by following specific cases in order to verify a mathematicalexpression, and in this context they performed several actions, such aspattern recognition, seeking the relationship between two variables, andmaking conjectures. They have performed three types of actions, namelyverification, explanation and abstraction during the proving of amathematical expression. Moreover, they have provided some argumentswhich were not accepted as proof, by offering experimental, intuitive or illogical justification. On the other hand, it has been observed that themiddle school mathematic teachers thought in the same way that theirstudents thought while proving a given mathematical expression.Discussion, Conclusion and Recommendations: As a result of this study, it hasbeen found that students had difficulties in proving mathematicalstatements; they preferred to use experimental proofs and mostly adaptedan inductive approach. On the other hand, the proving tendency of theteachers was mostly at a verification and explanation level; they have asimilar structure of thinking with their students in the process of provingmathematical expressions. Reasoning and proof should be thefundamental aspects of mathematics teaching, should play a significantrole in mathematical contents without taking it independently, and shouldbe developed in the earlier years. In addition, to what extent mathematicstextbooks and mathematics curriculum in each grade level support thereasoning and proof standards should be investigated.